Communication system



May 20, 1952 Filed May 9, 1946 F ICE-.4.

N. D. PRESTON COMMUNICATION SYSTEM Microphone Tone Genera-hr Radio TransmiH-er Radio Receiver TolK Lamp

4 Sheets-Sheet 3 TYPICAL CABOOSE (PRIMARY) STATION AnTenna TYPICAL LOCOMOT1VE(5ECONDARY) STATION Tone 5 Tco Microphone Loud 5p sake:-

- Radio TmnsmH Ter- Radio Recelver An+ennq fi'i'roke Bell INVENTOR.

N. D. PRESTON COMMUNICATION SYSTEM May 20, 1952 4 Sheets-Sheet 4 Filed May 9, 1946 .M zoFfim g 205:

LQ QUQ $2.200

INVENTOR.

moELo ZmPzuo gum CQ UE LB 0c Pat ented May [20 1952 CQMMUNIQATIQN' Neill),.Plfeston,jltochester, NgYz; assignor toiGeneraliRailway Signal Company; RliB$t81f;-N.'Y?

ApplicatiomMay-fi; 1946;;Serial 'Nm 66.8 353 8 la ms.

1'..- hisg-i ir c i i rrs atssatorr di omm ication ore art aule l toradio c uni- .sn d t erovid er ailro orel kas ecedin qu pm ;q fd m dzran a en: fi l- 915: larger :thanthe p fiesr ireo srat ne as l cthez h clefiqumme s1;

ln-rp oridina V diorcqmm ni at onwsystems fo rail-ncasisaand sim lav-ap lications 'irkYOlving 00111? n: nicationzbetween:anumbenof,movable vehic es nd a entrakofiice throughoutzan .exten-siv xteritory ereat r a hani h wop r tin :range :of :the radio equipment on;- the Nehicles, itsisprexpedient Q-p oyidesuitable.transmittingendreceivingapparatus ;at,-a p1urality of; "fixed. stationsv suitably located; and& spaced in:- that-territory to provide coverages-(wen :the different communication areas constitutingzthe:territoryafor desired communicat tion;lsogthattawehicleatsanypointin-theterritory isvin some one.of the:cemmunicationareasserved by: the fixectstations; The range-of. eaclr fixed stations isofi course-suflicient-tozpnoperlycoveir the communication areaaassigned to-it: Inthis way, anyvehicle'rim theterritory-ais withini-range'of one station;

Thesefixedistations wwhicheimtheoaseof araili road 1 communicationa systemare: conveniently tenrmedeway stations; are 1 connected to I the central ioffi'c'e -by a suitable two way communi'cation channel operating over'line wires or spaceradi ation, so: thatmessages reoeived at any fix'ed station:irom-a vehicle -within range of that-station will bet transmitted overthis communication channel to the central'oifioe, and any -me'ssage from the central office ma ce transmittedover the communication channel to the warious fixed stations to be broadcast by-these st'ationsto reach alt-vehicles in the communication territory.-

In such atypeof communication system; parti'cularly for=-use on railroads, it is desirable to provide 101* communication- *between the various vehiole's,-; as wel'l as between the central oflice and the vehicles? Iii-the case of'traincomrhunicatibn systems for railroads, it-is'also expedient'anddesira'ble' to provide for communioationioetweengthe front and -rearenda o'fthesame-train;

In such an organizatiomi when a given; vehicle a communication =-area is transmitting a messageto-the central'offica-it-is desirable that persons -on -other --vehicles other communication areas end-beyond the '0perating; :range of said; given vehicle should not also start-torsend-a message te-thecentral'pfiice and: mutilate or inter: fere witmtheme'ssage i1o'eing-transmitted to the rial 1 .39 7 v r:

2:". central. ofiiceg y' h ifirst vehicles lt ough itgis desirable to prevent such.-b'r;ea k;- in interference from other vvehicles up on amessage being sent to the-central oificathereis noreason why vehicles beyond the rangebfi-a vehi ole communicating- 'to the control office shouldl-not communicate with a h ot r; and wh ngtha ommunication ten-rior-y s xtens v t des r bl t rm teihsfin one communication:- area to commumcateaireely with each otherwhile,.-some ..vehicl e imanother communica ionareaissend nes 'messase bathe entral i ea In. vi w" heseu n i ra enaiandsnealsins e r l andtwithcut vat et i .tins to 1dat e or scope ft mesentz venti proposed to provide a 'commun-icatio; he tim .xund i onside tat o Whichrthe "var usvehicles .i ia cqmmumcatmn t e ri-tory are. pr vented. :from breaking 1111; upon transmissiom of-.s'a message from any oneo-f these ;vehicles .to. the gen?- tral ofiice, even though beyond the.:.0pe'ratin range. of .such vehicle, and, in which unrestricted communication is vpermitted between. vehicles which are; beyonmthez range zofz :said wehi-cle'eoom munioatingewithethecentral office. Stated more specifically it is proposed I toprovide suitable means :at each: of the plurality :of-- fiXed stations, whichis operated over the communicationchannel between these stations and -the central office; and whioh' is responsive to th'etransmission-to anyone of thesestations fromavehicle for renderin-g; all of the other stations --ineffecti-ve to respond. to messages for the-control ofiice that may be attempted byiotl'l'er vehiclesWithin-range of these other stations, so that -these other -vehicljesin other communication areas of the territer-ymay"notbreak-in upon-the transmission of amessage :t'o'the central ofiieeyicut; are not preventedfrom communicating-with each other.

The primaryobiect-'of---this-inyention i to provide --simp1e and efiective -n'ieans for providing these=-functi ohs and obtaining-theseobjectives. Various 1 other objects purposes; --characteristic features-and attrihutesof the invntionw'illiloe in part appa nt, and; in part pointed put as the pa t cu ar o an ation and stmqtura f, jparts. herein disclosed ia l y ical "em odime ts .of. .tthie invention-are described;

h u h thi linv ntionl is appli able to. 0n1: un a yst ms for yari usmi 'pp ss and u ,5, including different types f mobi e. n ts or l s. uchsas for communica onbetWem;a.-cen-' al .omcevandrc icei ars, truckslouses 311" Water: craft, or between'som'e central cities and pin;-

of the invention chosen for the present disclosure,

reference will be made to the accompanying drawings, in which like reference letters and numbers are employed, sometimes with distinctive prefix numbers, to designate similar and corresponding component elements of the system, and in which:

Figs. 1A and 1B illustrate diagrammatically and. conventionally the parts and circuits constituting the equipments at a central ofiice and a way station respectively, together with the line circuit constituting the communication channel between the central office and the various' way stations equipped the same as the typical way station shown in Fig. 1B.

Figs. 2 and 3 are explanatory diagrams illustrating the spacing of way stations on a rail- P road and the comparable operating range of a typical train-carried equipment.

Fig. 4 illustrates conventionally and diagrammatically the parts and circuits constituting the equipment for a typical primary station on a train, such as the caboose of a freight train.

Fig. 5 illustrates diagrammatically and con ventionally the parts and circuits for a typical secondary station on a train, such as on the locomotive or other motive unit at the head end of the train, and from which the operation of the train is controlled.

Figs. 6A and 6B are similar to Figs. 1A and 1B, and illustrate a modified form of control from the central office over the line circuit for initiating transmitting operation from the various way stations.

In order to simplify the illustration in the drawings, and also to facilitate an explanation and understanding of the'more important characteristics of the invention, various parts and circuits have been illustrated in accordance with certain conventions. Some component elements of the system, such as the conventional radio transmitters and receivers are indicated in block form; and other well known devices, such as microphones and loud speakers, have been illustrated schematically. To simplify the illustration of the electrical connections and circuits, arrows with the symbols and are used to indicate connections to the opposite terminals of a battery, or other suitable source of operating energy for the circuits, the direction of these arrows indicating a fiow of current into and out of the connecting wires. The various relays, which are assumed to be of the usual neutral type with armatures biased to the retracted position by gravity or spring means, are illustrated conventionally, with their windings in block form, and their associated contact fingers in a lower or inclined position when the relay winding is deenergized, and in the raised or horizontal position when the relay winding is energized, these contact fingers being shown either above or below the showing of the relay winding and indicated as connected thereto by dotted lines, as more convenient for the spacing and location of the parts.

Considering the general organization of the specific embodiment of the invention illustrated 4 and designed for train communication on railroads, it is contemplated that the communication between front and rear ends of a train, between different trains, and between trains and the central office will be carried out on the same radio carrier frequency in the megacycle band of frequencies at present allotted to such railroad use. Such relatively high megacycle frequency, in the order of 160 mc., as well as other practical considerations, make it expedient to employ traincarried radio transmitting and receiving apparatus of an efiective operating range which is ordinarily insufdcient to provide coverage for the entire territory or length of railroad over which communication with a central office is desired, such as a dispatching or engine division for a railroad. Accordingly, in order to provide the desired coverage, it is contemplated that a plurality of fixed way stations will be located along the railroad at intervals based upon the efiective operating range of the train carried equipments, and that these way stations will be connected to the central office for two-way communication by a line circuit, radio link system, or other suitable channel of communication. As illustrated, this channel communication between the central office and all of the way stations in the equipped territory is over a pair of line wires 4, used exclusively for this purpose or jointly with other power, telephone or telegraph facilities, in accordance with the usual practice in the communication field. Also, as shown, the communication over the line wires 4 is provided by a line carrier frequency involving suitable transmitters and receivers for this line carrier frequency located at the central office and at each way station. These transmitters'and receivers for the line carrier frequency are of the usual type and construction well known in the art and operate on a suitable carrier frequency for line circuits at the appropriate energy level dependent upon the operating conditions in accordance with well known practice. Since transmitters and receivers for this, purpose are well known, and the details of their construction and mode of operation are not material to the invention, such line carrier transmitters and receivers are illustrated diagrammatically in block form. It should also be understood that this line circuit between the central ofiice and the various way stations is provided with suitable terminations, as indicated at 5, and that the line carrier equipment is connected to this line circuit in the appropriate manner, such as by the transformers 6 and blocking condensers I or some equivalent shown, in accordance with recognized practice.

The line carrier transmitters and receivers provide two-way communication facilities to and from the central ofjce over what may be considered the wayside communication channel, which may be utilized to cooperate with the radio transmitters and receivers at the Way stations and on the trains and provide reception in the central ofiice of radio messages from the trains, and transmission of messages from the central office to the trains. Since the communication to and from the central oifice is assumed to be on the same line carrier frequency (as shown), and the system is normally in the condition shown for receiving messages from trains, it is necessary to provide additional distinctive control over the line circuit to enable the operator at the central office to set into operation the radio transmitters at the way stations.

In the embodiment of the invention shown in Figs. 1A and 13, this manual control by the opsweaters;

central :office. Referring to Fig.1l3, adirect-cur rent :relay. :2DC is connected across the line circuit in series with' a-suitable choke H, or-filter.

In-'connection with a traincommuni'cation' system for-railroads, chosen-for the purpose of illustratmgaspecific embodiment of the invention, the communication between the front and rear ends of the same train; between trains; and between trainsand the way stations, is provided by-a twoway radio telephone system of theusual and'well known: space radiation type, employing eitheramplitude orfrequency modulation, and operatingona carrier-frequency available for such pur pose; presumably'in the megacycleband. Since the structureand operation of radio telephone transmitters and receivers of this type are well known in the art, and the particular kind of transmitter or receiver is not'material for an understanding-of the present-invention, no attempt hasbeen made to-illustrate theparts and circuits for suchtransmitters and receivers, "which may be assumed to beef any suitable or conventional form, and are conveniently represented by rectan'gles designated radio-transmitter and radio-receiver. V

The fixedway stations are preferably provided with directional-antennas for transmission and reception, as indicated in Fig, 2, in the-'manner' and: fbr'th'e purposes-more fully explained in the prior :appIi'cation of F. B. Hitchcock, Ser. No. 6ll'flg3'59g filed June-'19 1945, Patent No; 2,469,555 granted May- 10, 1949. Generallyspeaking, the field patterns for these way'station antennas, indicated by-the dash lines in Fig. 2 limit the transmission of messages to thecontrol ofl'iceto one way station from any train located in the communication area on one side of that way station and extending to'the'next way station, so-that each communication area between successive way-stations is in effect covered or served by only onei-rway station. The effective operating range fora. train-carried equipment both to the front and..to.the:rear of the vehicle at the front or rear endsofthe train, assuming directional radiation in the -inter-.ests. of efficiency, as indicated by the field patterns in Fig. 3 is approximately thesame as-the spacingrbetween the way stations-so that a trai n stationat any pointin a communication area-iswithin eiiective range of the way station.

serving -that communication area... Itrshould be.

understoodtthat the spacing of the way stations is selected with: vdue regard to the variable :range of;.;train;stations.under different conditions, such. as.;weather= conditions and'the like,.as well' as taking into,-consideration the terrain; and other factors afi'ectingradiocommunication.

The, type .of;communication system assumed provides. for communication between v the front andarear 1 ends ,of the. same. train and betweentrains; and in; order .to avoid b'reak in interfer once fonsuchz communication, .anytrain-transmitting-a message docks out anapreventstra'nsmwsion from it any other-train station within range of the first train.

Brieflystated; this spacing of'way stationswith respect tothe operating range of train stations and the directional characteristics of the way star tion antennas, is employedso that as-trainstations-move along the railroad, any train station withincommu-nicating range of any given way station is likewise-within range of anyother-train stations within range ofthat same way-station,- and .is effective to lock. out-such other-train station against breaking in on any communication to that way station andthe-central olfice; Forexample, the radio station atthe front or rear endof a train at any point in the communication area of station A: (i. e-.-between=the way-stations- Aand B inFig. 2) has an operatingrange -td reach all other train-carried stationsin-this same communication area and capable ofreaching-the same way station A; and if one of these traincarried stations in this communicationarea should start communication viathe way station A to the central ofiice, its radio signal would reach alliothertrain stations capable of reaching thesame way station Aand-would be available toprevent break-in interference from such other train stations.

Since the trains have limited-operating range;- and the: equipped territory-may include: aplurality of communication areascorresponding directional radio antennas are all pointed iii-the sameedirection, but this is to be understood to be the same general clirectionsince railroads and other vehicular lines of trafiic do not always travel in exactly straight lines. For convenience in the disclosure, thecommunication areaassumed to be within range of any particular station has been indicated by:-a suitale'ie arrow and appropriate legend, and is shown as extend- I ing from what mightbedefined as an advantageouspoint'of reception adjacent one directional antenna up to a point'of -advantageous reception from the directional antenna of the-next way station. Since directional antennas are notperfectly asymmetridit will be -appreciated that this point of advantageous communication with respect to each antenna is actually a limited zone adjacent that Way station-antenna.

As above pointed out,-either amplitude or irequency modulationm'ay-beemployed.- However. the provision of frequency modulation has certain advantages because of its station-discrim'- inating characteristics. that a radio receiver of the frequency modulation type is capable of maintaining reception from that station which has the highest signal strength to the exclusion of'other stations having a lesser signal strengthalthough transmitting on the -same carrier frequency channel. In the situation where two stationsaretransmitting on It is well understood the same carrier frequency and are being received at the same time by a receiver, that station which has the higher signal strength is received in preference to the other due to the characteristics of the limiter stage in a frequency modulation type of radio receiver. This selectivity characteristic usually operates to a marked degree whenever the signal strength for the two transmitting stations as received at the receiver is in the ratio of two-to-one, but such characteristic is still somewhat effective even when smaller ratios are involved. Thus, it will be appreciated that the passage of a train station from the effective radio field or communication area of one station to another will receive one station in preference to the other on each side of a point of equal signal intensity of the two stations. But since a frequency modulation receiver tends to maintain reception of the station last tuned in while the signal intensity is still sufficient to be eifective, the actual point of changeover from one station to the other may vary from the point of equal radio signal intensities. Since the train under such circumstances will be moving, it is believed that the change from one communication area to another will be practically unnoticed especially since the transmission from all wayside stations is the same message from the central office. Various other characteristics of a fre quency modulation radio communication system are particularly adaptable to this type of organization.

In Fig. 2, the radio equipments have generally been designated RE and each includes a radio transmitter and receiver as more specifically shown in Fig. 1B for the typical way station A. These equipments are governed by the office controlled relays DC to determine when the normal condition for reception is changed to a condition for transmission. Each radio equipment has associated therewith the line carrier equipment designated LCE in Fig. 2 and including a line carrier transmitter and a line carrier receiver as more specifically shown in Fig. 1B.

Each of the way stations are assumed to be alike, so that the description of the apparatus and organization of a typical way station will be sufficient for an understanding of all of the way stations. It might be noted, however, that the way stations illustrated in 2 have their different devices designated by letter reference characters made distinctive by the use of the preceding numerals 2, 3 and 4 for the respective way stations A, B, and C.

Referring to Fig. 1A, the equipment in the central office includes the line carrier transmitter having a microphone associated therewith, a line carrier receiver having a loud speaker and a carrier operated relay CR associated therewith,

a push-to-talk button TB, and a transmit relay TR. The transmit relay TR is normally deenergized, so that the line carrier receiver is normally connected to the line wires 4 through back contact 15 of relay TR and through the coupling transformer t and blocking condensers l. Another contact it of relay TR serves to connect a source of plate potential to the line carrier receiver to render it active for reception. When the transmit relay TR. is energized, then the back contact 16 is opened to render the line carrier receiver inactive and the front contact IE is closed to render the line carrier transmitter active; and at the same time the contact 15 disconnects the line carrier receiver and connects the line carrier transmitter to the coupling transformerfi rendering the line carrier transmitter eifective for the transmission of a voice message spoken into the microphone.

The energization of the transmit relay TR is effected by the actuation of the talk button TB, which is of the self -restoring type, but when actuated connects through back contact I! of the carrier responsive relay CR to the transmit relay TR to cause it to pick up its contacts. Of course, if the line carrier receiver is receiving a carrier signal, then the carrier responsive relay OR is picked up, so that the operator cannot efifect the energization of the transmit relay TR. However, assuming that the relay TR can be effectively picked up, then its back contact. I0 is opened, removing energy from the line circuit formed by the line wires 6, which results in the deenergization of the office controlled relays DC at the several way stations, as for example, relay 2DC of way station A in Fig. 1B. This ofiicecontrolled relay 2DC governs the radio equipment at the way station, as will be presently described, it being sufiicient for the present to know that these relays DC for the several way stations are connected in multiple through their respective chokes H to the wayside line wires 4 and are normally supplied with direct current from the battery source 9 in the central oflice through back contact I 9 of relay TR. The choke 8 is included, in series with the battery source 9 for the purpose of preventing this application of direct current to the line wires 4 from becoming a short circuit to the carrier signals impressed upon the wayside communication channel by any one of the line carrier transmitters. It is, of course, obvious that this is essential because the line carrier receiver at the central office must be capable of receiving a message transmitted from a line carrier transmitter anywhere in the territory regardless of the normal application of direct current to the wayside line wires. The chokes l l at the various way stations are provided for the same reason.

Referring to Fig. 1B, illustrating a typical way station equipment, conveniently designated way station A, the radio receiver is normally connected to the directional antenna through front contact I8 of relay ZDC. Also, the radio receiver is rendered active by reason of the connection of its plate supply through front contact IQ of relay ZDC. On the other hand, if a message is to be transmitted from each way station, the operator at the central onice actuates the talk button TB which results in the removal of direct current from the wayside line wires 4 thus deenergizing all of the office controlled relays DC to render the various radio transmitters active and the associated receivers inactive. Specifically, at the typical station A, this results in the deenergization of relay 2DC disconnecting the antenna from the receiver and closing its connection through back contact I8 to the associatedradio transmitter. Similarly, the closure of back contact l9 connects the plate supply to the radio transmitter after removing it from the radio receiver. In this way, the radio receiver is normally rendered active or effective to receive a radio message, but can be selectively rendered inactive and the associated radio transmitter rendered active subject to the manual control of the operator in the central office.

It might be noted here, that the tubes of the radio transmitters and radio receivers as well as the line carrier transmitters and the line carrier receivers, are assumed to be continually lighted, so that the selective switching operation of these different devices can effect their immediate re- 9'? sronser t rthan waiti s'for e m n cessar foratheheating of thevarious vacuum tubes involved,

Normally, the coupling transformer 15 is connected'throughback contact "20 to the line carrier receiver, and the plate supply is connected through back contact 2| so asto render such line carrier receiver effective-for the receptionof a message transmitted from the central ofiice over the wayside communication channel. The reception of the line carrier signal itself causes the energization of a carrier responsive relayZCB, while the'voice messages imposed upon thecarrier current are demodulated to give an audio output, which, as indicated by the wire 22, is transferred to the radio transmitter in such a way as to beused for the modulation of the'radiocarrier-frequency signals transmitted by theradio transmitter assuming, of course, that. it is rendered active by the deenergizati-on of the relay 2DC.

Orr-the other hand, if a radio signal is being'received, itmay not only be modulated by the voice messaga'butalso by distinctive tone of a suitable character'that does not interfere with the voice message, This distinctive tone is passed through the band pass filter indicated, and 'rectifiedby a suitable rectifier unit2R, the output of which passes through the back contact 23 of the carrier responsive relay ZCRfor the energization of the transmit relay 2TB. In other words,'if a radio signal is received having the distinctive tone thereon, the transmit relayZTR is energized and its contacts picked up to render the associated line carrier receiverinactive and the associated line carrier transmitteractive. The voice messagefrequencies are passed through a suitable connectionindicated by wire 24 to the input of theline carrier transmitter to effect the modulation ofthe line carrier frequency that is imposed upon the wayside communication channel. Suitable filter apparatus 25 is employed to prevent the distinctive tone from being passed over connection 24. to the line carrier transmitter.

brief,-.itcan be seen that the audio output a circuit of the radio receiver indicated by wire 2 goes to the linecarrier transmitter and similarly that the audio output circuit of the line carrier receiver is connected to the radio transmitter.

so that the audio components of the voice messages received by either receiver can act upon theradio carrier frequency in the case of the radio transmitter and the .line carrier frequency in the case of'the line carrier transmitter to modulate-suchfreguencies in such away that .the

messagerece'ived at the way station willbe-transmitted' away from that'way station by the proper associated transmitter.

For the purpose of simplicity, it may be assumed that the modulated radio carrier frequency *signa'ls'are' received and amplified and converted int'o'audio frequencies in the same manner as a 'conventionalra'dio receiver, andthat this audio frequency, instead of operating .a loud speaker, acts (like a microphone) to modulate the frequencytransmitted by the line carrier current transmitter, although it should be understood "tral oi'fic'emay be effective to transmit a message r transmitter "and a radio receiver.

10 to -all of the waystations which are respectively effective to transmit corresponding radio messages. Also, the reception of a radio message for the office at any particular way station throughout the communication territory, results intheretransmission of that message over "the wayside communication channel for reception at'the'centraloffice. In-this connection, itlmay be noted that the message imposed upon the wayside communication channel by theline carrier transmitterof any way station is also areceivedat'all other way'stations, butat such.- other way station it isineifective to operate the radio transmitters, since atthistime the radio transmitters cannotbeeffective due to the energized condition of "their associated office controlled relays DC. -.IIo-wever, the reception of suchcarrier-signalat' each of the various otherwaystations causes the energization of their carrier responsive relays corresponding to relay ZCRof the typical way station of- 'Eig. 1B, opening "back contact'23 at those various waystations. Thus, in the event of the reception of a radio Isignal at any one of such other way stations from-some mobileunit also endeavoring to transmit-armessage tothe central office, thetransmitrelay -at that'station cannot be picked up and'cause a carrier signal tobe transmitted interfering with the'message already on the'wayside-communication channel. Inthis way,'the wayside stations are so interlocked-'that'the firstwllay station receiving a radiosignal having the distinctive toneimposed thereon, gains control of the wayside communication channel to 'the "exelusion of theother waysidestations and-thus prevents possible interference from someother station.

It might-be notedin this connection'that'the reception of'a message at the central o'fliceis received by .thefline carrier receiver and "dc-.-

modulated to be reproduced bythe conventional loud speaker, while the carrier frequency itself 'is'eifective-to cause the energization of the carrier responsive relay CR opening backconta'ct il. This prevents the central ofiice'operator from interrupting the reception of a message.

Typical primary station.Referring to Fig. '4,

atypical caboose or-primarystationhas been diagrammatically illustrated as including a radio 7 The radio receiver has the conventional loud speaker 2 conneoted-to its output. and also a carrier'responsive relay designated 40R, which'is-picked up'by the reception of a ccarrier "signal regardless of whether it is modulated or not. Normally,'.-the

radio receiver is rendered active .by reason of :its

connection to-the plate supply through back-contact 3B of relay 4TR,and also by-its connection to its associatedantenna through back-contact 3i of this same. relay QTR.

When the transmit relay 4TB is ,picked up, either-as a result-of manual control, or fort-he transmission of check pulse, thee radio 1 receiverais renderedinactive, and the radio transmitter is rendered active. This is becausethe-plate supply is 1 connected through front I contact 7 30 :to the radio transmitterand the. antenna is also shifted by contact "3| tothe radio transmitter.

A talk button ETT is for establishingcommunication between the caboose. station .and another train or 1 its associated locomotiue s tation; whereas the talk button lTCOis for estab-- lishing communication between the caboose station and the central office via some way station. Bothof these buttons are of theself-restoring 11 type and the actuation of either button causes the energization of the associated button repeating relay GBP by an obvious circuit.

The picking up of the relay 4B? closes a circuit from through front contact 32 of relay 4BP, back contact 33 of carrier responsive relay 40R, windings of transmit relay 4TB, to This energization of the transmit relay 4TB renders the radio transmitter active, and since the button repeater relay 4BP closes a front contact 34, the microphone is rendered effective for the transmission of a voice message. The trainman' is advised that it is proper for him to talk, namely, that his transmitter is rendered active, by reason of the illumination of the talk lamp 4TK by reason of a circuit closed from (-1-) and including front contact 35 of relay 4TB, front contact 36 of relay 4BP, through lamp 4TK, to

If the trainman desires to talk with the central oflice, as above mentioned, he actuates the talk button 4TCO instead of the button Z'IT, which in addition to energizing the repeater relay 4BP, also closes back contact 31 to cause operation of the tone generator associated with the radio transmitter. This tone generator is assumed to be of any suitable electronic type which will produce a tone preferably of the supersonic range that will not interfere with regular voice transmission and reception over the radio carrier signal. However, it should be understood that since the frequencies required to transmit voice messages may include only the limited range between 500 and 2,500 cycles per second, it may be feasible to employ some frequency which is not supersonic but which is filtered away from the voice message circuits at the receiving station. In any event, this distinctive tone, regardless of its frequency, is superimposed upon the radio frequency signal in addition to the voice modulating audio frequencies, and is filtered out by a suitable band pass filter at the receiving wayside station for the purpose of actuating a tone responsivetransmit relay at such station, as previously described.

The release of either talk button which has been actuated, will restore the primary station to its normal condition in which it is intermittently transmitting check pulses. More specifically, two pulsing relays designated 4A and 4B are employed to provide the timing characteristics for the intermittent transmission of these check pulses. As indicated in the drawings, the relay 4B is energized through back contact 38 of relay 4A and its front contact 39 is energizing relay 4A which has not yet responded to such energization. As soon as relay 4A responds to its energization, a circuit is closed for the transmit relay 4TB from and including front contact 4! of relay 3A, front contact 4| of relay 43, back contact 32 of relay 4BP, back contact 33 of relay 40R, windings of transmit relay 4TH, to This energization of relay 4TH causes the radio transmitter to transmit a check pulse signal without modulation sinces the front contact 34 of relay 4BP is open; and this check pulse persists only for a short interval of time measured by the release of the relay 4BP and the release of the relay 4TB. In other words, as soon as the relay 4A is picked up, it opens back contact 38 to deenergize the relay 4B which immediately opens front contact 4| to release the transmit relay 4TB to end the check pulse transmitted. The length of the check pulse may be suitably selected in accordance with the various operating times for the relays involved in the complete system organization, but itiis contemplated that the duration of such pulse will be in the order of a half second or the like. The release of the relay 4B, of course, deenergizes the relay 4A, which is slow to release for a suitable period to determine the spacing between successive check pulses. This release period of the relay 4A may be of any suitable time, but is contemplated as being in the order of three to four seconds, at the termination of which back contact 38 will be closed for picking up the relay 4B and initiating another check pulse. It will, of'course, be appreciated that any suitable times may be employed in these relays so as to give the desired length check pulses with the desired spacings, but it has been found practical to employ check pulses of approximately a half second duration with spaces between pulses of approximately 3% seconds. Y

These check pulses transmitted by the typical caboose or primary station, are received by the associated secondary station to provide a check on the communication between the primary and secondary stations as will be discussed in greater detail presently. It is desired to point out here that this intermittent operation of these pulsing relays 4A and 4B is arrested by the actuation of either talk button i'IT or 4TC'O, since the energization of the button repeating relay ABP closes front contact 42 to maintain relay 4A energized throughout the transmission from this primary station. This, of course, leaves the relay 4B deenergized so that upon the termination of the transmission of a message, a time interval corresponding to the usual length of spaced between successive pulses is imposed upon the pulsing organization before the first check pulse is transmitted. As will be presently explained, this provides a suitable control for restoring the check lamp at the associated secondary station.

It should also be noted that a similar operation occurs following the reception of a radio message either from a secondary station or from a wayside station within the communication area in which the. primary station is then located. This is because the reception of a radio signal causes the carrier responsive relay iCR to be picked up closing front contact 43 to maintain the pulsing relay 4A energized, the same as during transmission under manual control. A similar organization of check pulse control has been shown and described in my prior application Ser. No. 580,224 filed February 28, 1945, now Patent No. 2,408,878 dated October 8, 1946, and no claim to this subject matter is intended to be included in this application.

Typical secondary station.A typical locomotive or secondary station organization has been shown in Fig. 5 as including a radio receiver and a radio transmitter. The radio receiver is provided with the conventional load speaker connected to its output circuit and is also provided with a carrier responsive relay ECR in asimilar manner as explained in connection With the radio receiver of Fig. 4. The radio transmitter has a conventional microphone provided for modulating its radio frequencies signal, and also an associated tone generator with the same characteristics and for the same purposes as explained indetail in connection with Fig. 4.

The radio receiver is normally active because the antenna is connected thereto through back contact 50 of relay 5TB, and the associated plate :13 's'upply 'islconnected' -:through1 back: contact 151: of relay 5TB. However; the-:actuation of :either the transmit button 1 BTT' or 1 the transmit button STCO results in the energization of the-transmit,

lamp 5'I K to advise the op'erator" or trainman 1 that histransmitter is ready 'foriaavoioe message. The 'transrnit button ETT is for communication with the prima'ry' station 501 other trains as the case may be, whereas the actuation of thetalk button STCO is for establishing communication with the central 'ofiice by "the-way .ofthe particular wayside station" within the communication area i-n Which the train is then located. This -is efi'ectedby the closure of back contact '55 or: the talk button BTCO which renders the tone generator-active to cause a distinctive tone to be superimposed upon the radio carrier signal for rendering the associated: wayside radio r ceiver' e'fiective to act upon its associated line carrier transmitter, :aspreviously explained.

Under normal conditions, the typical locomotive station is within range of its associated caboose statiomand is intermittently receiving the check pulses above explained. Between the successive check pulses; theclosureiof' back. con

tact 56 energizes the back. contact" repeater relay I 53? through' an obvious circuit; whereas the reception of a *check' pulse causes the carrierrelay fiCRtobemomentarily' picked u-p: to energize the front contact'repeater relay-EFF. Thebackcontact-repeater relay 5B1? isjustslightly slowacting, i. e. sufiici'ently to maintain its' ccnta'ct fi'a' "picked up during "the reception of the usual length check pulse. On the other hand, the 'frontcontact' repeater-relay EFF- is sufliciently 'slow" releasing to remain picked up between succ'essivec'heck pulses with a suitable margin, but, of course; this relaywill drop'away' if the carrier responsive relay 5GB, is deenergize'd fora period substantially greater than the time interval between check pulses. Thus; the intermittent operation of the carrier responsive relay SCR main-v "tains' bothoi the relay 5F? and taBP picked up which completes' an energizing circuit fro'm through front c'on-tacttfl of relay EFP, front contact 'fil of relay BP, check lamp EEK and windings of belPEB-R. in' m-ultiple to During thereception of a message either from the primary station or the then associated way- 'side istation, the carrier responsive relay 5CR'is pickedwup which maintains the "front contact repeaterrelay 'fiFPpicked' up, but-allows the back contactrep'eater relay 5BP to'release andextinguish the check lamp "K and cause the single .bell 5133. to sound. Immediately following such war ning, the operator expects to hear a voicemessagethrough theimedium of the loud speaker,--and'the failure toreceiVesu-chra inessage adviseshim of some'failurein the communication system.

:At the termination of theireception of a;mes sage, there :is 'a ;ce'ssation of the carrier signal fora time period corresponding to the usual space :betweenasuccessivecheck .p'ulses, which is sufificient to releaseithe relay fiCR andree'nergize 14 the-back cnntact ater'relay 5.131; Sinoerthe pickup period of the relayBBP isirelativelyauick, although the relay is 'slightl'yislowreleasing, the check lamp 56K is immediately illuminated at the termination of the reception 'of avoice message.

During the transmission of'a'message from the locomotive or'secondary station of-Fig. '5, =there are no check: pulses received since the-carrier signal transmitted f rom this :secondary :"station causes the transmission of check pulses" to cease the prim'ary station, "and also because the secondarystatien :could not receive check pulses during its transmission even ifsuch o'heckpul'ses were available. Thus, the :carrierresponsive-re lay ECR remains deenergized closing back: con- .tact 56 .to.maintain1the backcontact repeaterrelay .EP'B picked up, but, .of course, the front. contact 5,6 is ropen'iand' would allow the front contact repeater relay tFP to become deenergize'd. CHOW- ever; under theseconditions thecheck lamp 561; should remain -illuminated since there is no' message being received. To accomplish this; aistick cireuit'is provided for thefront con-tact repeater relay 5F? closedfrom and including front contact 59 of relay 5TB, front contact'fiii of relay 15FP, winding of relay 'fiFP, to Thus, as long as the transmit relay "5TB is "picked up "during the transmission from the secondary station, the front contact repeater relay 5F? is maintained energized along with the'bacl: contact repeater relay 5133?. This maintains the check lamp 50K illuminated by reason of its-obvious energizing circuit.

With this understandin 0: the general organ-izationand functioning of "the system,: it "is believed that further characteristic features of the system willbebest understood by considering diiTerent-operating conditions encountered in the application of the present invention to'a railway.

Typical system operations Normal conditions.-As previously expla'medain connection with Figs. 1A and 1B,'and'a1so referring to Fig. 2, the Wayside stations-arenormally :in a' condition for the reception of radio signals because the central office normally assumes an inactive condition. The various-train stations, such as a typical caboose'or-primarystationiand at associated typical locomotive or secondaryfstation; may be moving anywhere within the communication territorynand each station-will be located Within the communication areaof at least one wayside station -for reasons previously texplained in connection WithEig. 2.

These Var-ious train stations-or mobile'sta'tions are also normally conditioned for the reception of radio signals; but the primary train stations will, of course, be transmitting theircheckupulse signals at spaced intervals. Since the :check pulses are of relativelyshort duration'and the spaces between the pulses is substantiaL eachprimary station is in conditionto receive more-than half of the time so-that transmissionfroma-wayside-station would, of course, immediatelytbreak in upon the intermittent transmission of check pulses from the. primary stations, in -'a-manner presently to be discussed in greater detail. It is desired'to notahowever, that this transmission of check. pulses from eachfprimary 'stationais eriective to maintain the checkxlamp at assooi- :atedsecondary: station illuminated.

These cheek pulses transmitted by each: primaryistation will also bereceived by the adjacent T5 operation of the associated line carrier transmitter since these check pulse signals are not modulated by the distinctive tone required to render the line carrier transmitter of a wayside station active. With this organization, it is then apparent that even though there may be a plurality of primary train stations located in different communication areas throughout the communication territory, and even though such primary train stations may be transmitting check pulses at different intervals out of synchronism, they cannot prevent the central ofiice from rendering all of the wayside stations effective to transmit at the same time whenever the operator at the central ofiice desires.

It might be noted also that if two or more primary train stations are within range of each other, the fastest operating check pulse apparatus acts to synchronize the check pulses apparatus at the other primary train stations in such a way that this relatively fast operating primary train station is the only one withinits particular range which transmits check pulses. This is because the fast operating check pulse transmitting apparatus places each check pulse carrier signal on the air which is received at each of the other stations closing the front contacts 43 for each pulse prior to transmission of a check pulse signal from that station. This acts to maintain the relay 4A energized and prevent such apparatus from transmitting a check pulse of its own. This feature of self-synchronization between the different primary stations has been disclosed and described in detail in my above mentioned prior application Ser. No. 580,224 filed February 28, 1945, now Patent No. 2,408,873 dated October 8, 1946, so that no claim to this subject matter is intended to be included in this application.

Transmission from central office. Let us assume that the operator in the central oifice desires to transmit a message to any one or several of the train stations within the communication territory. To do this, he actuates the talk button TB (see Fig. 1A) in the central office which energizes the transmit relay TR and opens back contact to remove direct current energy from the line wires 4 as supplied by the battery 9. This removal of direct current energy from the line wires 4 causes the deenergization of the oilice controlled relays ZDC, SDC, ADC, etc., as shown in Figs. 2 and 1B, which renders the radio transmitters at each of the several stations effective for the transmission of messages.

As soon as a way station has its radio transmitter rendered active, its carrier frequency signal is transmitted so that it is received by each of the train stations within its communication area. This causes the energization of their respective carrier responsive relays, such as relays 4GB. and 50R of Figs. 4 and 5, to lock out or prevent such mobile train stations from transmitting. In addition to this lock-out operation, the picking up of the carrier responsive relay 40R at a primary train station acts to cause the cessation of the check pulse transmitting apparatus at that station; and also at each secondary train station the continued energization of the carrier responsive relay 5GB. results in the extinguishment of the associated check lamp 50K and the sounding of the associated single stroke bell 5BR.

With these conditions established, the communication organization is now in condition for the operator at the central office to transmit any message he may desire by speaking into the microphone. This message is transmitted to each of the several way stations by modulating the line carrier frequency which is received at each of the several way stations and is demodulated by their respective line carrier receivers. The audio output of each line carrier receiver is transferred or fed to its associated radio transmitter to cause the modulation of the radio carrier frequency signal being transmitted. Since this operation occurs at each of the several wayside stations, it is apparent that every mobile train station located within the communication territory will receive the message. It is also assumed that the operator begins the message transmission immediately following the actuation of his associated talk button TB, so that there will be no substantial gap following the extinguishment of the check lamps CK at the variou secondary train stations and the reception of messages at such stations.

As soon as the operator at the central office completes his message, he releases the transmit button TB which renders the associated transmitter inactive and also reapplies direct current from the battery 9 to the line wires 4. This reestablishes normal conditions at each of the Wayside stations causing a cessation of the transmission of a carrier frequency signal from such stations. This allows each primary train station to initiate transmission of check pulses after a time interval equal to the regular time interval or space between successive check pulses. This, of course, causes the check lights at each of the secondary train stations to be restored immediately upon the cessation of the reception of the wayside carrier signal.

Transmission from primary station to central cfiice.Let us assume that the trainman at a primary train station desires to communicate with the central offioe. To do this, he actuates the talk button ATCO assigned to this purpose instead of the talk button lTT intended to in-- itiate transmission to another train station (see Fig. 4). This actuation of the talk button 4TCO closes back contact 31 to render the tone generator associated with the radio transmitter in condition for modulation of the radio carrier frequency with a distinctive tone frequency as soon as the radio transmitter is rendered active by the transmit relay 4TB. With reference to Fig. 4, it can be seen that the actuation of the transmit button dTCO causes the energization of the repeater relay GBP which in turn causes the energization of the transmit relay 4TB as previously explained. The closure of front contact 34 of relay 4BP renders the microphone effective, and the closure of front contact 42 arrests the operation of the pulsing relays 4A and 6B. As soon as the transmit relay 4TB is picked up, the radio carrier frequency signal is transmitted; and this carrier signal is modulated with the distinctive tone so that a distinctive signal is received by the particular wayside station next adjacent as Well as any mobile train stations within receiving range. The energizetion of both of the relays lBP and 4TB. causes the illumination of the talk lamp ATK so that the trainman is advised that the system is in readiness for transmission of his message.

At each of the secondary train stations, within range of the particular primary station then transmitting, the check lamp such as lamp ECK, is extinguished and the station is in a condition for the reception of any message that is spoken into the microphone at the primary train station. Also the reception of the carrier frequency signal at such secondary train station looks out that station by opening back contact 52 of the carrier responsive relay 50R so that the trainman thereat cannot interfere with the communication already initiated by the primary train station.

It should be understood, that all train stations beyond receiving range of the primary train station then transmitting are in no way affected; and if desired, separate communication between such stations can be independently carried on, as .will presently be discussed.

The reception at the adjacent wayside station (such as the typical station of Fig. 1B) of the carrier signal with the distinctive tone superimposed thereon as transmitted from the primary train station is effective to cause a current of the distinctive tone frequency to pass through the band-pass filter 26 which is rectified by the associated rectifier 2R. to cause the energization of the transmit relay 2TB, at such way station. This energization of the transmit relay ZTR, of course, renders the associated line carrier receiver inactive and the associated line carrier transmitter active for transmitting a line carrier frequency signal over the wayside communication channel. The reception of such line carrier signal at the central oilice and at each of the other wayside stations causes the picking up of the associated carrier responsive relays and prevents any transmission from either the central ofiice or any wayside station.

For example, it is apparent that the picking up of the relay CR at the central oflice opens back contact I! and prevents the operator at the central ofiice from energizing the transmit relay TR. Similarly, if it is assumed that some other wayside station is transmitting a line carrier signal,'and the typical way station A of Fig. 1B is. receiving such line carrier signal, it will be apparent that the energizationof the relay R opens back contact 23 and prevents the transmit relay 2TB. from being energized even though some train station within its communication area should transmit a radio carrier frequency signal with the distinctive tone superimposed. In other words, the transmission of a radio carrier frequency signal modulated with a distinctive tone by a first train station in one communication area results in the transmission of a line carrier frequency signal by the associated wayside station to the central ofiice and to each of the other wayside stations. This prevents a second train station in any other communication area, and not directly receiving from the first train station, from interfering with the transmission of a message to the central oifice by the first mentioned train station. In the case under consideration, the first train station is assumed to be a primary train station, but the same operations and conditions exist with respect to secondary train stations.

Assuming that the operator or trainman, at the primary train station being considered, transmits his message by speaking into the associated microphone, it will be received only by the train stations within range and by the wayside station within the communication area of which it is then located. When the message is received at the way station, the radio receiver at that station acts to demodulate the received signal and pass the audio output through the low-pass filter and connection 24 to the associated line car rier transmitter (see Fig. 1B). The filter 25 will be of a suitable character, such as a low-pass filter to allow only the audio message to pass to the line carrier transmitter for modulation of its output and transmission to the central oifice, where it is received and is again demodulated and reproduced in the loud speaker. The low-pass filter 25 prevents the distinctive tone transmitted by the primary train station (or secondary train station) from passing through the connection 2d to the line carrier transmitter; As previously mentioned, this tone may be suitably selected above the audio frequencies employed for voice transmission and filtered out of the voice reproduction circuits without inter fering with the voice message.

At the end of the message, the primary station, of course, restores to its normal condition for the transmission of check pulses, and the removal of a radio carrier frequency signal with distinctive tone frequency at the receiving way station, of course, allows its tone responsive relay such as relay 2TB, to restore the wayside equipment to its normal condition.

Since communication with'the central oiiice by one train station looks out all others in the manner just described, it may happen that a train station in one communication area will endeavor to transmit to the central ofiice while a train station in another communication area is already using the wayside communication channel, but, of course,will receive no indication of this fact; For this reason, it may be expedient for a train; station to first call the central oifice for permis sion to transmit its message. The transmission of a permission message by the central oflice for a particular train station to proceed to transmit will, of course, be received by all train stations and thus advise them to desist from endeavoring to use the wayside communication channel. With such a procedural setup, it may also be desirable for the central oflice to transmit a line-clear message when a particular train station has completed the transmission of its message, thus advising all train stations that the wayside communication channel is again clear. It is, of course, to be understood that any desired procedures may be followed in employing the present invention in actual practice.

Transmission of secondary station to central ofiice.-Assuming that a trainman at a secondary train station, such as typically shown in Fig. 5, desires to communicate with the central office, he similarly actuates the talk button ETCO instead of the associated talk button 5T'I provided for communication between train stations. This conditions the tone generator by the closure of back contact 55 and energizes the transmit relay 5TB directly. This transmit relay 5TB closes front contact 53 to render the associated microphone efiective as well as closes front contact 54 to render the talk lamp STK illuminated. The transmission of the radio carrier frequency signal modulated with the distinctive tone frequency from this secondary train station is, of course, received at the adjacent wayside station and any other train station within range. At the train stations that may be within range the carrier acts to lock out such station and also at the primary train stations acts to stop the check pulse transmitting operation. This is necessary so as to prevent any interference by check pulses with the message to be transmitted to the cen tral office via the wayside station;

The reception of such carrier signal with the distinctive tone superimposed thereon, of course, causes the proper response of the tone responsive relay at the wayside station to render the line mitter, but acts to filter or block any of the distinctive tone frequencies from the retransmitted message.

In this way, it is possible for the operator at the central office to obtain the same control over the wayside stations by the use of a distinctive tone as is provided by the use of a direct current control shown in Figs. 1A and lB. Insofar as the typical operations of the system are concerned in connection with this modified form of the invention, it is to be understood that the wayside stations and the mobile train stations have the same characteristics and accomplish the same functions and features previously described.

Having described a system providing for communication between mobile stations as well as between the mobile stations and a central ofilce, as a specific embodiment of the present invention, it is desired to be understood that the forms of the means shown have been selected to facilitate the disclosure of the invention rather than to limit the number of forms which it may assume; and, it is to be further understood that various modifications, adaptions and alterations may be made to the specific embodiment shown to meet the requirements of practice without in any manner departing from the spirit or scope of the present invention except as limited by the appended claims.

What I claim is:

1. In a radio communication system providing two-way communication between at least two mobile stations and also between the mobile stations and a plurality of fixed way stations on n the same carrier frequency, radio transmitting and receiving means at each of said stations, each of said receiving means at said mobile stations being normally in a condition to receive radio carrier signals and also capable of reproducing a message imposed upon such a carrier signal, but each of said receiving means at said way stations being normally in condition to receive radio carrier signals although being ineffective to reproduce any message, a tone generator at each of said mobile stations associated with the radio transmitting means at that station, check pulse means at one of said mobile stations for automatically rendering its transmitting means active and its receiving means ineffective at spaced intervals to cause the transmission of spaced check pulse signals, means including a carrier responsive relay at said one mobile station for rendering the associated check pulse means ineffective at that station upon the reception of a radio carrier signal from any mobile or fixed station, indication means at the other mobile station maintained inactive only upon the continued reception of said spaced check pulse signals by its associated radio receiving means, manually operable means at each station effective when actuated to render the associated receiving means ineffective and the associated transmitting means active to send out a radio carrier signal, said manually operable means at each of said mobile stations being distinctively operable to at times render its associated tone generator effective to superimpose a tone frequency on the radio carrier signal being sent out by its associated radio transmitting means, and tone responsive means at the way station distinctively responsive only to the reception of a radio carrier signal having imposed thereon said tone frequency for allowing any message imposed upon such carrier signal to be effectively reproduced, whereby the check pulse signals transmitted by said one mobile station and the transmission of radio messages between said mobile stations in either direction are not effectively received .at the way station.

2. In a communication system for providing communication between a plurality of movable vehicles and a central ofiice, radio transmitting and receiving means on each vehicle having a limited range, fixed stations at intervals having radio transmitting and receiving means and spaced at intervals to provide a coverage corresponding with the range of the vehicle equipments, a line circuit connecting all of said fixed stations and a central office, transmitting and receivingmeans at the central oifice and at each fixed station operating on a line carrier frequency, the equipment at each fixed station being normally in condition to receive a :radio signal or the line carrier frequency, relay means at each fixed station connected to its radio receiving means and energized by the reception of a radio signal from any vehicle within range for disconnecting the associated line carrier receiving means from the line circuit and connecting to said line circuit the line carrier transmitting means, means at each fixed station responsive to a line carrier frequency transmitted from any other fixed station or the central ofiice for preventing energization of said relay means, and means at each fixed station manually controlled from the central oifice by energization of said line circuit distinctive from said line carrier frequency alone for rendering the associated radio transmitting means eifective to send out a message transmitted by carrier over said line circuit from the central ofiice.

3. In a train communication system for providing radio communication between trains and between a control ofiice and trains at different locations in an equipped territory, radio transmitting and receiving means on each train having a limited range, manually operable means on each train effective only if no carrier is being received by its receiving means for rendering the normally active receiving means ineffective and its transmitting means effective to transmit signals from the train, way stations at intervals along the railroad having radio transmitting and receiving means with uni-directional antennas directed in the same direction along the railroad, said way stations being spaced to conform with the respective radio range of the trains, means including transmitting and receiving means at the central oiiice and at each way station and operating on a distinctive carrier frequency for providing a two-way communication channel between said central office and all of said way stations, relay means at each way station governed over said communication channel for rendering the carrier frequency transmitting means at that way station ineffective to transmit to the central ofiice radio messages received from a train when any other way station is being used for communication with the central ofiice by some other train, and means at each way station manually controlled from the central office over said communication channel by an energization distinctive from said carrier frequency alone for causing all way stations to send out a radio message transmitted from the central office over said communication channel.

4. In a communication system for providing two-way communication between a central office and a plurality of movable vehicles, radio equipment on each vehicle including transmitting and receiving z-m'sans operating on same carrier srecuency, aplurality of 'fixed'stations'each having -ra'dio transmitting and receiving :means operating on the same carrier frequencyiandrspaced at intervalsto provide "a coverage corresponding *with'the edective range of the radio eouipmenton the vehicles, means-including a carrier .frequency ransmifltcr and receiver at each station and constituting kcommunica tion channel 70011116513- :in'g all .of :said fixed stations and a central :omce for two Way lcommunic'ation'between :said central 01 1 10813115 said fixed stations, relay :controlm'eans ."a each :fixed station responsive to the reception '0' a :radio message ifrom .a vehicle within range :of "that station for rendering the carrier ireouency transmitter at that station effective to act upon-saidcommunicationchannelin response :tozmessages received .byits radio VreoeizingImeans, means at each fixed station governed over :said 'cmmnunication channel and acting whenever a vehicle establishes communication with any fixed station for irend'eringinefiective asaid relay control :means at all other fixed stations independently :of the reception :of radio mesages, and manually operable means in the central .office eifectiveonly if no communication with the central :ofiicefrom any vehicle :is then in street for transn'ritting a distinctive control over said communication channel to "all fixed stations LtOLren'der their radio transmitters efiective to send to all vehicles :a message transmitted by :carrier over said communication channel from the control zofice.

:ilkehicle equipment for radio "communication systems comprising, a transmitter (and a receiver operating on thesame carrierirequency, nreans manually operable to :render said receiver inactive and :said transmitter a'ctive. :a relay rersnonsive to the reception of a carrier frequency by-rsa id receiver either modulated or unmodurlated, a :slow release relay energized when said carrier operated relay is energized, a stick circnit for. said slow release relay including .its own frontcontact, said rstick circuitbeing closed when manually operable :mean'sis actuated to render the transmitter s'active if said slow release zrelas then picked up, and indicating means maintained inactive :only so long as said slow release (relay 'is energized.

6. A communication system of the character described comprising, a plurality of spaced fixed stations each having a radio transmitter and receiver, a central :oifi'cc, means constituting a communication channel zbetween said central officenand said stations and acting to transmit :from ;:all stations a carrier frequency modulated with the message ."from the central o'ifice, radio equipment on each of a plurality of vehicles .ineludinga transmitter and a receiver operating on the same carrier frequency as the station transmitters andreceivers, pulsing means on certain of said vehicles normally acting to transmit :at regularly spaced intervals pulses of unmodu- "lated 'carrier frequency provided no carrier freuency modulated or unmodulated is then being received by that station, indicating means on other vehicles, and means for maintaining said "indicating means on each of said other vehicles inactive only :so'long as carrier frequency pulses mehein'g received at spaced intervals, said indicating means being rendered active by the reception of a sustained carrier frequency either from la'fixed station'or from another vehicle.

In acommunication system of the character described for a central since and :a plurality of vehiclesga plurality of fixed stations,v a line on cult 'ccuine'cting all-cf said fixed stations "with n central office, radio equipment including transmitting and receiving means at each fixedzstation and on each of a plurality of vehicles operating on the-same carrier frequency, the radio equipmerit on each vehicle being normally in condition to receive and being manually operable only provided no :carrier'f-requency'ris being-received from some other source to transmit 'a carrier frequency with or without anaau'diblecontrol tone along with the message for communication with a fixed station within range or with some other vehicle within range, controlmeans at each fixed station responsive to therec'eption ofsaid control lton'eonly from .a vehicle for providing a line roarri'er :on :said line circuit to he modulated with the smessage from vehicle, means :at each fixed station responsive to said line carrier'forrendering its controlmeans unresponsive to the :reception oi a control toneby its associated receiving means, and means manually operable in a control office provided no line carrier is then being received for transmitting a distinctive control over said line circuit, and means at each fixed station responsive to said distinctive control on said line 0' cult for rendering active the radio transmitters at all of the fixed stations to permit a message from the central office to be sent-to all vehicles.

.In a communication system providing radio communication between a plurality of movable ehicles and a plurality of lfixed station's connected with a central oflice, radio equipment on each vehicle including transmitting and receiving means of limited range operating on the same carrier frequency, circuit means manually controllable for rendering said radio transmitting means at times efiective to transmit a message with or without a distinctive signal, a "first relay means controlled by said radio receiving means when a carrier frequency is being received to prevent said manually controllable circuit means from rendering said radio transmitting means effective, a plurality of fixed stations located at intervals corresponding with the range of said vehicle equipments, each of said fixed stations having radio transmitting and receiving means operating on said carrier'frequency,means providing a communication channel connecting all of .said fixed stations and said central o'fiicc. control means at each fixed station including a control relay and an energizing circuit 'for such relay controllable by the associated radio receiver when said distinctive signal is received for causing the received message to be repeated onto said communication channel ior transmission to the central office, and a second relay means at each fixed station governed over said communication channel and operated wheneversaid communication channel is in use .for transmission to the central ofiice from any fixed station andineluding a contact insaid energizing circuit-opened when said second relay means is operated for preventing operation of said control means by reception of said distinctive signal, said second relay means being eiTecti-ve on said controlmeans at all fixed stations other than the station then receiving from a'mov-ahle vehicle without rendering the associated radio transmitter active, whereby any vehicle beyond the range of a'given vehicle communicating with the central zoifice i's unalole to establish an interfering communication'wi-t'h the central ofiice but :may sc'ommimicate at any time with other vehicles within its Number Name Date range. 1,740,621 Nyguist Dec. 24, 1929 NEIL D. PRESTON. 1,891,299 Anderson Dec. 20, 1932 2,061,699 Fox Nov. 24, 1936 REFERENCES CITED 5 2,064,958 Taylor et a1. Dec. 22, 1936 The following references are of record in the 2,248,746 Dams July 3, 1941 me f this patent; 2,265,056 Bowers Dec. 2, 1941 2,393,291 Clark Jan. 22, 1946 UNITED STATES PATENTS 2,407,846 OBrien Sept. 17, 1946 Number Name Date 2,469,555 Hitchcock May 10, 1949 1,678,672 Espenschied et a1. July 31, 1928 2,490,061 Jensen et a1 Dec. 6, 1949 

